1. Field of the Invention
The present invention relates to a high data rate electrical connector and cable assembly and, more particularly, to a connector/cable assembly which includes a connector or connectors attached to a cable having multiple twin-ax wire pairs.
2. Description of the Related Art.
The Quad Small Form-Factor Pluggable (QSFP) connector is a connector capable of achieving a 40 Gb/s data rate (QDR, quad data rate, with the governing standards specifying a bandwidth of approximately 5 GHz) using InfiniBand, Ethernet, or other networking protocols. To achieve these high data rates, particularly with respect to 40 Gb/s Ethernet, crosstalk between the differential pairs within the connector must be reduced. Reducing crosstalk allows for a higher signal-to-noise ratio and reduces the amount of processing needed to achieve these higher data rates.
A QSFP cable assembly is a twin-ax cable with a QSFP connector module attached to both ends. The cable generally has eight twin-ax differential pairs (four transmit and four receive) with a drain wire for each pair. Each of the sub-cables (differential pair conductors and respective drain wire) typically has a conductive foil which is in contact with the drain wire, and there typically is a braided conductive shield around the eight sub-cables. A printed circuit board (PCB) in each connector is attached to the cable's differential pairs at the respective ends of the cable assembly, with four differential pairs and their respective drain wires connected to PCB terminals on one side of the PCB. The other four differential pairs and their respective drain wires are connected to PCB terminals on the other side of the PCB. The PCB terminals that connect to the drain wires are connected to ground planes in the PCB with vias (plated through holes) in the PCB.
One method of connecting the drain wire to the PCB is to attach it directly to the PCB by way of shaping the drain wire so that it bends around and ends up lying next to one of the differential pair wires, as shown in FIG. 1. Some problems that arise from this termination method include that the drain wire is attached to the PCB next to only one of its differential pair signal conductors which creates an unsymmetrical relationship between the ground (drain wire) and its differential pair signal conductors. Having a non-symmetric relationship between two conductors of a differential pair and ground can lead to common mode generation which ultimately creates crosstalk.
U.S. Patent Application Publication 2010/0029104, incorporated by reference as if fully set forth herein, describes a SFP+ (small form-factor pluggable) connector pair manager for use in securing a twin-axial cable to a connector printed circuit board. The pair manager provides a symmetric termination between two conductors of a differential pair and the drain wire/ground. However, the SFP+ (small form-factor pluggable) connector typically includes only two twin-ax terminations on one side of the SFP+ connector PCB.
Currently for a QSFP connector the maximum twin-ax cable outer diameter that can fit into it is a cable where the individual signal conductors are 24 AWG, although 24-30 AWG are used for different lengths of cable assemblies, and smaller than 30 AWG are also acceptable. A typical goal for QSFP cable assemblies is that for a given length, (maximum currently 7 meters for 40 Gb/s Ethernet, 5 to 6 meters for InfiniBand) the minimum wire size should be used while still meeting the insertion loss requirements. The form factor for the QSFP connector is set by the SFF-8436 standard, and one challenge with respect to fitting the cable into the connector is that it can be difficult to fit 24 AWG cable, which is used for the longer reach cable assemblies.